by Ken Task.
The method of upgrading called for copying back into new code plugins/addons from old code directory to appropriate/same locations in new code.
Those include themes.
In a 3.5.stock, themes directory had boost, bootstrapbase,clean, and more. Did you have any addiional theme for the 3.5.x?
Themes have changed in 3.7.x. *ONLY* boost and a new one called 'classic'.
By chance did the copy of old code themes include a theme that isn't compatible with 3.7.x?
The moodle environment check doesn't check plugins for compatiblity with higher version of moodle code.
Just one incompatible plugin can cause some very strange errors and send one off down a rabbit holes!
'SoS', Ken